1. Field of the Invention
This invention relates to the field of absorbent products including super absorbent polymers, especially for use by persons with urinary incontinence. More specifically, the invention relates to such products including a sensor or monitor which provides an indication when the absorption capacity of the super absorbent polymer has been exceeded.
2. Description of the Prior Art
There are numerous examples of wetness indicating devices which have been proposed for use in diapers and other absorbent products for use by persons with urinary incontinence. One example of a wetness indicator is represented by a class of hot melt adhesive products which include a wetness indicating component. U.S. Pat. Nos. 4,743,238, 5,035,691, 5,066,711 and 5,089,548 disclose various hot melt adhesives with wetness indicators, many of which would be suitable for use in a capacity monitor according to the present invention. These hot melt products are in wide use in disposable diapers today. In conventional use, they are applied in the form of a narrow straight bead, a thin ribbon or a spiral bead to a water impervious backing sheet and are in direct contact with, and in liquid communicable relation with, an absorbent layer of fluff. Tests conducted on several commercially available disposable diaper products including such conventionally applied hot melt adhesives with wetness indicators demonstrate that the wetness indicators provide a positive indication of wetness, i.e., a color change in the hot melt, when no more than 15 milliliters of a 0.9% NaCl solution was deposited thereon. Even 5 milliliters of such solution was sufficient to effect a color change in some of the conventional applied hot melt adhesives with wetness indicator. The color change occurred over relatively short lengths, i.e., between 1/4 inch and 2 and 1/2 inches, of the beads or ribbons of hot melt adhesive with wetness indicator at the low volume levels of 5 to 15 milliliters.
U.S. Pat. No. 3,731,685 discloses a moisture indicating strip which is designed for use with diapers and other absorbent products. One end of the strip is attached to the inside of the diaper or the like and the other end, which is impregnated with a color change chemical, extends out of the diaper. Liquid is transported by capillary action from inside the diaper or the like to the color change chemical outside the diaper.
U.S. Pat. No. 3,759,261 discloses diapers having a layer with a pattern printed thereon and one or more layers which mask the pattern when dry. When wetted, the masking layers no longer mask the pattern which becomes visible through a backing sheet, thereby providing a wetness indicating function.
U.S. Pat. No. 3,952,746 discloses a humidity indicator strip incorporated in a diaper including a water impervious backing sheet. A portion of the strip extends through an opening formed in the backing sheet.
U.S. Pat. No. 4,022,211 discloses a diaper including a wetness indicator comprising a layer of water soluble or water dispersible coloring agent applied to the water impervious backing sheet. When dry, the coloring agent is clearly visible through the backing sheet but, when wetted, the coloring agent is dispersed and is no longer visible through the backing sheet.
U.S. Pat. No. 4,231,370 discloses a wetness indicator in a diaper structure. The wetness indicator is positioned between a water impervious backing sheet and an absorbent core and comprises a flexible pH-change/color-change coating which is preferably applied to the backing sheet and visible therethrough. The preferred coating material comprises first and second latexes, an acid buffer and bromo-phenol blue.
U.S. Pat. No. 4,287,153 discloses a diaper including a saline water indicating material positioned between a backing sheet and an absorbent core, and visible through the backing sheet. The preferred indicating material comprises a water-insoluble polymeric ion exchanger and a water-insoluble polymeric exchanged ion indicator in intimate contact with each other.
U.S. Pat. No. 4,507,121 discloses a diaper including a wetness indicator comprising a color change chemical which is isolated from the absorbent portions of the diaper so as to prevent contact between a wearer of the diaper and the color change chemical. Means are provided for supporting one way capillary action to transport liquid from the absorbent portions to the indicator to trigger the color change.
U.S. Pat. No. 4,704,108 discloses a wetness indicator comprising two metal strips incorporated in a diaper and circuitry for detecting changes in the electrical conductivity in absorbent portions of the diaper due to the presence of moisture therein.
U.S. Pat. No. 4,705,513 discloses a wetness indicator for diapers. The indicator comprises indicia printed in waterproof ink on a paper carrier strip. The paper strip is positioned adjacent to a water impervious backing sheet and the indicia is masked by the strip so long as it remains dry. When wetted, the carrier strip no longer masks the indicia and it becomes visible through the backing sheet to indicate wetness.
U.S. Pat. No. 4,738,674 discloses an absorbent product including a moisture indicator which provides an indication of wetness at a point remote from where liquid is actually deposited on the product. The indicator includes a wicking strip for transporting liquid from a point where it is deposited to an edge of the product where it contacts a moisture indicating substance which can provide a visual indication, such as a color change, that it has been wetted.
U.S. Pat. No. 4,834,733 discloses a diaper including an actuating member which shrinks when wetted and it is connected to an indicating mechanism so that, when moisture contacts the actuating member, it shrinks and displaces the indicating mechanism providing an indication that fluid has been deposited in the diaper.
U.S. Pat. No. 5,078,708 discloses a diaper comprising a transparent or translucent, water impervious backing sheet and an absorbent core. Opening are provided in the core so that urine or stools deposited in the diaper are visible through the backing sheet to indicate the need for a change.
Super absorbent materials have been incorporated in commercially available diapers and the like, for many years. U.S. Pat. No. 5,072,687, the disclosure of which is incorporated herein by reference, discloses several varieties of absorbent diaper products including super absorbent materials. Known super absorbents absorb large amounts of liquid and liquid absorbed in super absorbents can not be squeezed back out of the super absorbents. This is to be contrasted with fluff which is composed of cellulose or the like fibers. Fluff can absorb many times its weight in liquid but liquid absorbed in fluff can be squeezed back out of the fluff. In a diaper, squeeze out of urine from fluff can cause leakage and prolonged skin contact with urine soaked fluff can cause damage to healthy skin and inhibit healing of damaged skin.
Each super absorbent polymer has a "speed" which is used herein to mean the rate at which the super absorbent will absorb liquid. The speed of a super absorbent depends upon many factors including its composition, the quantity of liquid it has already absorbed and the amount of liquid available to be absorbed. For a given liquid, each super absorbent also has a "free swell capacity" which is used herein to mean the maximum amount of liquid which unrestrained super absorbent particles will ultimately absorb when exposed to an excessive amount of a liquid, i.e., one which far exceeds the capacity of the super absorbent relative to that liquid. A given super absorbent with a given capacity for a given liquid will have a different, somewhat lower capacity for that liquid when the super absorbent is contained in, for example, an absorbent core of a diaper. The term "in-situ capacity" will be used hereinafter to refer to the volume of a given liquid which is absorbed by super absorbent associated with an absorbent core containing at least one other, different absorbent material, such as fluff, within thirty minutes after the liquid is deposited slowly in a single spot in the absorbent core. The in-situ capacity of super absorbent in such an absorbent core does not include the capacity of the fluff or other absorbent material in the core; the term refers only to the capacity of the super absorbent to absorb a given liquid, under the recited conditions, within thirty minutes of the time the liquid is deposited.
Some super absorbents are fast and some are slow. In granular form, commercially available super absorbents take approximately one to three minutes to absorb their free swell capacity of water, when unrestrained and exposed to large amounts of water. When super absorbents are incorporated in absorbent cores with other absorbent materials, such as fluff, they absorb liquid more slowly than this, largely because they are in contact with less liquid. All super absorbents have a given speed when dry, and, as they absorb liquid, the speed and remaining capacity of the super absorbent decreases.
Some of the characteristics discussed in the preceding paragraph are illustrated in FIG. 1 in respect of super absorbents contained in absorbent fluff cores in diapers. FIG. 1 is a plot of data collected from tests made of a commercial version of several medium sized SlimLine.RTM. diapers manufactured by Principle Business Enterprises, Inc. In the test, many different quantities of a 0.9% NaCl solution were deposited on SlimLine medium size diapers. For each quantity of solution, the diapers were tested for skin wetness at different intervals of time after the solution was deposited. In the skin wetness test, a medium size SlimLine diaper was placed, with its water impervious backing sheet facing downwardly, on a level surface. In the center of the diaper, a given quantity of a 0.9% NaCl solution was deposited at the rate of 7.0 milliliters per second. Absorbent paper toweling, cut into three inch by five inch rectangles, was stacked, the stack was weighed and its dry weight was recorded. At various time intervals after the 0.9% NaCl solution had been deposited on the diaper, a previously weighed stack of paper toweling was placed on top of the diaper and a three inch by five inch, rectangularly shaped five pound weight was placed on top of the paper toweling. After fifteen seconds, the weight was removed from the paper toweling and it was reweighed. The weight of the dry toweling was subtracted from the weight of the wet paper toweling to give a skin wetness number corresponding with the weight of 0.9% NaCl solution which was released by the diaper into the paper toweling. The time which elapsed before the skin wetness number was 0.0 grams for a given quantity of liquid was plotted for several different quantities of liquid and the results are shown in FIG. 1. The medium sized SlimLine diapers which were tested had 12 grams of Aridall 1465 super absorbent material, available from Chemdal, in the fluff layer. At relatively low volume levels, the speed of the super absorbent in the SlimLine diapers is relatively fast, as reflected by the steep slope of the plot on the left side of FIG. 1. At higher volume levels, the slope of the plot is not as steep and this reflects the well known phenomenon associated with super absorbents used in absorbent products, namely, the speed of a super absorbent material decreases as it absorbs liquid. As the capacity of the super absorbent is used up, the speed decreases and the slope of a plot like that shown in FIG. 1 approaches zero. It will be appreciated that the in-situ capacity of the super absorbent in a medium SlimLine diaper for synthetic urine is approximately 400 to 450 milliliters. The plot in FIG. 1 provides a good way to approximate in-situ capacity because, for example, the fluid volume of about 425 milliliters at thirty minutes elapsed is almost entirely absorbed within the super absorbent material, whereby the skin wetness number is 0.0 because the super absorbent does not release liquid under pressure. The volume of 285 milliliters associated with a skin wetness of 0.0 after ten minutes is significant, in terms of the medium SlimLine, because it is preferred that skin not be in contact with a diaper with skin wetness in excess of 0.0 for more than ten minutes.
In a SlimLine diaper, which comprises a substantial quantity of absorbent cellulose fluff (70 grams) as well as the 12 grams of super absorbent polymer, liquid is immediately absorbed into the fluff component and, over time, the liquid is withdrawn from the fluff and absorbed by the super absorbent polymer. So long as the in-situ capacity of the super absorbent is not exceeded by the quantity of liquid, the fluff layer will, within thirty minutes, dry out in the sense that it will preferentially give up absorbed liquid for absorption by the super absorbent polymer in the diaper.
It has been determined that urine absorbed in and held by super absorbent material in a disposable diaper is innocuous to a wearer and skin adjacent to and in contact with a diaper which contains urine held by super absorbent material (and no urine held in a fluff layer) is not damaged by such contact. This is true even for relatively large volumes of urine such as 100 or 200 milliliters. Prolonged exposure of skin to urine, either direct exposure to urine or exposure to urine which is absorbed in fluff, can damage healthy skin by causing a rash or other reaction while prolonged exposure of already damaged skin can inhibit healing and aggravate the damage to the skin.
As noted above, when only 15 milliliters of 0.9% NaCl solution is applied to commercially available diapers including conventionally applied hot melt adhesives with wetness indicators, the wetness indicators change color signalling the need for a change of the diaper. This was true, even for commercially available diapers which included super absorbent materials. It will be appreciated that such use of hot melt adhesives with wetness indicators in a SlimLine diaper, or other diaper including super absorbent material, would provide, in many cases, a premature indication of the need for a diaper change because:
1. it takes approximately two minutes for the super absorbent polymer in medium SlimLine diapers to completely absorb 100 milliliters of 0.9% NaCl solution (0.0 skin wetness) but the super absorbent has a remaining in-situ capacity to completely absorb, in ten minutes, another 200 milliliters of 0.9% NaCl solution; PA1 2. in the two minutes that it takes for 100 milliliters of 0.9% NaCl solution to be absorbed out of the fluff into the super absorbent, that quantity of 0.9% NaCl solution in the fluff would trigger a color change in conventionally applied hot melt adhesives with wetness indicators.
While this phenomenon might increase sales of SlimLine diapers and other products incorporating conventionally applied hot melt adhesives with wetness indicators, it would also unnecessarily increase the burden on already overburdened landfills as well as the burden on care givers who tend to the changing of absorbent products used by persons with urinary incontinence. So, there is demonstrated a need for a capacity monitor which is capable of providing a reliable indication of when the in-situ capacity of super absorbent material has been exceeded, without providing a premature indication, such as when there is substantial remaining in-situ capacity in the super absorbent.